Typically, a packet switched wireless access network contains AN (Access Network) elements connected to a PDG (Packet Data Gateway). A PDG serves an MN (Mobile Node), via the AN to which the MN is attached. The PDG may provide connectivity for the MN to the packet data network and may perform the function of a mobility agent to support network layer mobility for the MN. A handoff of the MN may change the MN's Layer 2 point of attachment from one PDG to another PDG (either within the same wireless technology or across different wireless technologies), resulting in a new link layer and network layer connection being established between the MN and the new PDG. The problem of IP mobility is commonly addressed by the Mobile IP protocol; however, such a handoff can cause a significant disruption in real-time traffic due to issues such as handoff latencies, loss of transient packets caused by the Layer 2 handoff from the medium, and the Layer 3 protocol latency caused by Mobile IP protocol, for example.
These latencies impact the performance of real-time/streaming applications (e.g., audio and video) and other near real-time applications that are based on existing link layer technologies (e.g., cellular and WLAN (wireless local area network)) as well as emerging wireless technologies such as 802.16/WiMAX. A transient loss in connectivity or lack of response can lead to false indications and subsequent performance degradation of such applications. The total blackout period experienced by an application during handoff is typically a function of the Layer 2 handoff delay and, when network layer handoffs are involved, the Layer 3 delay also. Layer 2 latency is usually the time between link layer detachment from the old access point and reattachment at the new access point. Layer 3 latency is usually the time to re-establish L3 connectivity (e.g., the MN arriving on the new link and detecting and completing registration with Layer 3 agents).
The performance degradation can be apparent to the user and can impact the user's seamless mobility experience. For example, a user of a streaming video application running on a mobile station during a handoff may experience a visible loss in video frames and distortion. Moreover, in TCP-based applications, a loss of packets during handoff may be falsely interpreted by the TCP as a network performance problem like congestion, resulting in the TCP protocol resorting to back off mode (with congestion control and re-transmits), thereby reducing the overall TCP throughput. Thus, the effects of the handoff on TCP may not be limited to the handoff but may last well beyond the handoff interval.
Improved handoff techniques such as Fast Mobile IP handoff have been proposed in the IETF. These help improve the network layer handoff latency by helping overlap the Layer 2 and Layer 3 handoff processes. However, their performance is still limited by Layer 2 handoff latency. It appears that even with the Fast Mobile IP (FMIP) handoff scheme, there is still an inevitable packet loss and the negative impact of handoff on TCP-based applications remains unresolved.
Therefore, a need exists for an apparatus and method for link layer assisted handoff that addresses at least some of the above mentioned limitations of both standard and Fast Mobile IP handoff techniques and that can improve the mobility experience of an end user running an IP-based application (such as VoIP, image, video, streaming media, PTT, etc.).
Specific embodiments of the present invention are disclosed below with reference to FIGS. 1-5. Both the description and the illustrations have been drafted with the intent to enhance understanding. For example, the dimensions of some of the figure elements may be exaggerated relative to other elements, and well-known elements that are beneficial or even necessary to a commercially successful implementation may not be depicted so that a less obstructed and a more clear presentation of embodiments may be achieved. Simplicity and clarity in both illustration and description are sought to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. One of skill in the art will appreciate that various modifications and changes may be made to the specific embodiments described below without departing from the spirit and scope of the present invention. Thus, the specification and drawings are to be regarded as illustrative and exemplary rather than restrictive or all-encompassing, and all such modifications to the specific embodiments described below are intended to be included within the scope of the present invention.